1. Field of the Invention
The present invention relates to systems and corresponding methods for removal of fixed pattern noise (FPN) from sensor images. More specifically, the present invention relates to systems and corresponding methods employing feedforward shunting for removal of fixed pattern noise (FPN) from sensor images.
2. Description of the Related Art
Focal plane arrays (FPAs) are used in various applications to capture images for subsequent processing. As disclosed in U.S. Pat. No. 5,903,659, the teachings of which are incorporated herein by reference, air-to-air missiles have employed infrared sensors with FPAs to capture infrared images of a target being tracked. The FPA typically contains n×n infrared detectors, where each detector has a slightly different sensitivity to infrared radiation. This non-uniform sensitivity yields fixed pattern noise (FPN), which manifests itself in the image, resulting in some pixels which are too bright and some which are too dark. In short, the fixed pattern noise (FPN) manifests as sensor-fixed artifacts induced by nonuniform response of the focal plane array (FPA). The nonuniform response causes the FPA output to be spatially varying even when illuminated by a uniform source.
Some missile sensor processors have employed rotating optical elements and complimentary techniques to reduce FPN. These techniques have often introduced significant scene artifacts in the output image, which cause the missile tracker to mis-track the target. Efforts to reduce the scene artifact growth have resulted in insufficient fixed pattern noise reduction under certain flight conditions.
Other techniques for suppressing or reducing FPN have also been implemented. It will be noted that techniques such as those based on stored nonuniformity correction factors can correct for the nonuniform array response under static conditions. But dynamic inputs, such as changing photon flux induced by dome heating, requires dynamic or adaptive non-uniformity compensation. Because FPNs are fixed on the focal plane, and hence are temporally correlated, they present a more difficult false alarm problem than that presented by temporally uncorrelated temporal noise (TN).
Yet another technique for reducing FPN employs coning, i.e., selective readout of the FPA following a repetitive, multi-frame pattern. It should be mentioned that a FPA coning system allows the offset terms to suppress only the (FPA fixed) FPN without significantly reducing the target intensities whose locations in the FPA are changed from frame to frame. However, there are several problems with the traditional GEN2 ADNUC design. For example, the use of an aggressive transfer function creates deep black holes, sometimes referred to as black ghosts. These holes are caused by the inversion of the outside scene objects and targets. As a result, the intensities of any scene objects and targets nearby, or overlapping with, the black holes will be reduced and distorted.
In addition, the feedback additive process of the offset terms will often cause an increase in temporal noise (TN). Current ADNUC systems, e.g., the GEN2 ADNUC system, are not designed to remove temporal noise (TN). Traditional ADNUC systems use an additive feedback technique wherein a correction-offset term is accumulated from an error-term which is generated from the output (de-meaned) image. The correction term is then subtracted from the next input image. The feedback coefficients, i.e., the non-linear transfer functions, can not be set too aggressively if one desires to avoid an increase of TN and black-hole artifacts. Therefore, depending on the feedback coefficients, it may take many image frames for the conventional ADNUC system to reduce the FPN from an initial high value to a low equilibrium value.
Finally, as discussed in copending patent application Ser. No. 09/175,213, and 09/275,223 (the teachings of which are incorporated herein by reference) the feedback accumulative process will slow down the adaptive process for reducing FPN and non-uniformity.
Thus, there is a need in the art for a technique for removal of fixed pattern noise (FPN) from sensor images without introducing significant scene artifacts. It would be highly desirable to provide a system and corresponding method employing feedforward shunting techniques for removal of FPN from sensor images without increasing temporal noise associated with the sensor images. There is a particular need for a system and corresponding method employing feedforward shunting for removal of FPN from sensor images which would reduce FPN from an initial high value to a relatively low equilibrium state over a minimum number of frames. Ideally, the system and corresponding method would employ feedforward shunting techniques for removal of FPN from sensor images without increasing the magnitude of black-hole artifacts, i.e., without either reducing or distorting the target signals.